The morphology of the developing cranial base may well be related to the transposition of the palatal shelves. A comparison of the cranial base is being made in strains of mice which have differential susceptibilities to the cleft palate inducing teratogen 6-aminonicotinamide (6-AN). The A/J strain is susceptible whereas the C57 BL/6 strain is somewhat resistant to 6-AN. The change in morphology of the cranial base, the H3 thymidine and 5 35 sulfate uptake as visualized by autoradiography, and the alteration of mandible and other craniofacial structures are being examined.